Polyaxial Orthopedic Fastening Apparatus

ABSTRACT

An orthopedic fastener is designed to attach an implant to bone in such a manner that, until the fastener is tightened, the implant may be rotationally adjusted against the bone. The implant may have a semispherical bone apposition surface that permits polyaxial rotation of the implant against the bone. The orthopedic fastener has an interpositional member and a compression member. The interpositional member may be a split ring with a conical exterior surface that mates with a conical surface of the implant. The compressing member has a threaded bore that engages a proximal end of a fixation member implanted in the bone such that, in response to rotation of the compression member, the interpositional member is sandwiched securely between the implant and the compression member. The conical surface of the implant compresses the interpositional member about a semispherical surface of the compression member, thereby restricting rotation of the implant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication having the Ser. No. 11/063,941 filed on Feb. 22, 2005 andclaims priority to U.S. application Ser. No. 10/860,778 filed Jun. 2,2004 which carries Applicants' docket no. FSI-2 NPROV and is entitledSPINAL FACET IMPLANT WITH SPHERICAL IMPLANT APPOSITION SURFACE AND BONEBED AND METHODS OF USE, which are incorporated in their entirety herein.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to systems and methods forattaching implants to bone, and more specifically, to a polyaxialorthopedic fastening apparatus particularly useful in the field of facetjoint replacement.

2. The Relevant Technology

Orthopedic medicine provides a wide array of implants that can beattached to bone to alleviate various pathologies. One unique challengein orthopedics is to provide implants and fastening devices that areadaptable to a variety of bone morphologies. Each patient will have adifferent bone structure; accordingly, it may be necessary to allow foradjustable positioning of an implant with respect to the bone so thatthe implant will be positioned to perform its function.

For this reason, a number of fixation systems have been invented thatenable variation of the angle between the implant and the fastener.Although such fixation systems generally permit adaptation to the bonemorphology of a patient to provide secure anchoring of the implant tobone, they are generally somewhat limited in the types of adjustmentthey permit with respect to the bone. Accordingly, such fixation systemsmay not be usable with a number of implants that require morecomprehensive adjustability. Furthermore, many known implant fixationsystems are complex due to the presence of several parts, or due to theneed to perform several steps to utilize them to attach an implant tobone. Yet further, some known implant fixation systems are expensive,and require the use of unusual tooling. A need exists in the a forimplant fixation systems and methods that alleviate the foregoingshortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is an exploded, perspective view of a vertebra with an apparatusaccording to one embodiment of the invention, with the apparatuspositioned for attachment to the vertebra.

FIG. 2 is a caudal, section view of the vertebra and the apparatus ofFIG. 1, with the orthopedic fastener of the apparatus in the unlockedconfiguration to permit adjustment of the orientation of the implantagainst the bone.

FIG. 3 is a caudal, section view of the vertebra and apparatus of FIG.1, with the orthopedic fastener in the locked configuration to restrictrotational motion of the implant against the bone.

FIG. 4 is an exploded, perspective view of a vertebra with an apparatusaccording to one alternative embodiment of the invention, with theapparatus positioned for attachment to the vertebra.

DETAILED DESCRIPTION

The present invention advances the state of the art by providing systemsand methods that can be used to anchor orthopedic implants to bone in amanner that provides a high degree of implant adjustability, simplicity,and ease of use. The present invention can be used in any orthopedicprocedure, but may have particular utility in the field of facet jointreplacement to alleviate back pain resulting from traumatic,inflammatory, metabolic, synovial, neoplastic and degenerative spinaldisorders. The configuration and operation of selected embodiments ofthe invention will be shown and described in greater detail withreference to FIGS. 1 through 4, as follows.

In this application, the terms “compression member” and “interpositionalmember” are used broadly. A “compression member” generally is a memberthat receives a compressive force. An “interpositional member” generallyis a member, at least part of which is designed to be positioned betweenat least two other members of a system. Deformation of one part “tolock” another part generally relates to deformation of the first part insuch a manner that the first part physically impedes relative motionbetween the two parts, rather than simply supplying frictional force toprovide locking. However, the present invention encompasses embodimentsthat provide locking solely via one or more frictional forces.

“Polyaxial” rotation is rotation that can occur about at least two axesthat are not parallel to each other. “Triaxial rotation” is rotationabout three perpendicular axes. Triaxial rotation is equivalent torotation about a point, because free rotation about any axis of a 3Dcoordinate system is the same as rotation that is not limited to anyaxis in the system.

Referring to FIG. 1, a perspective view illustrates an apparatus 10according to one embodiment of the invention, in use with a vertebra 12,such as an L5 lumbar vertebra of a human spine. As shown, the vertebra12 has a body 18, which is generally disc-shaped. The vertebra 12 alsohas two pedicles 20 extending from the body 18, and a posterior arch, orlamina 22, that extends between the posterior ends of the pedicles 20 tocouple the pedicles 20 together. The vertebra 12 also has a pair oftransverse processes 24 that extend laterally from the pedicles 20, anda spinous process 26 that extends posteriorly from the lamina 22.

The vertebra 12 also has a pair of superior facets 28, which arepositioned toward the top of the vertebra 12 and face generallymedially. Additionally, the vertebra 12 has inferior facets 30, whichare positioned toward the bottom of the vertebra 12 and face generallylaterally. Each of the pedicles 20 of the vertebra 12 has a saddle point32, which is positioned generally at the center of the juncture of eachsuperior facet 28 with the adjacent transverse process 24.

The superior facets 28 of the vertebra 12 articulate (i.e., slide and/orpress) against the inferior facets (not shown) of an adjacent superiorvertebra (not shown) to limit relative motion between the vertebra 12and the superior vertebra. Thus, the combination of each superior facet28 with the adjacent inferior facet defines a facet joint (not shown).Accordingly, two facet joints span the distance between each adjacentpair of vertebrae. The inferior facets 30 of the vertebra 30 are part ofother facet joints that control motion between the vertebra 12 and ailadjacent inferior vertebra (not shown) and/or the sacrum (also notshown).

Each of the facet joints may be covered by a capsule (not shown)containing a fluid (not shown) that reduces wear of the facets 28, 30and facilitates articulation. Additionally, layers of cartilage (notshown) may cover the facets 28, 30 to further reduce wear and facilitatearticulation. These anatomical structures, as well as the variousmuscles, ligaments, and nerves of the spine, will not be depicted in theFigures to enhance the clarity of the disclosure. Such structures may beremoved or displaced according to known methods to provide the necessaryaccess to the vertebra 12.

As shown, a semispherical resection 34 has been formed on one of thesaddle points 32 of the vertebra 12. The semispherical resection 34 isshaped to receive an implant to replace the articular surface of one orboth of the adjacent superior and inferior facets 28, 30. Thesemispherical resection 34 permits relative rotation between the implantand the vertebra 12 about three perpendicular axes prior to fixation ofthe implant to the vertebra 12. The axes may be defined as shown byreference numerals 40, 42, and 44 in FIG. 1.

More precisely, the axes may include a first axis 40, a second axis 42,and a third axis 44. The first axis 40 is generally collinear with theaxis of the corresponding pedicle 20. The second axis 42 is generallyvertical (i.e., parallel to the axis of the body 18) and perpendicularto the first axis 40. The third axis 44 is generally horizontal (i.e.,parallel to the end plates of the body 18) and perpendicular to thefirst and second axes 40, 42.

The apparatus 10 includes an implant 50, a fixation member 52, and anorthopedic fastener 54, or fastener 54. The implant 50 is designed toseat against the semispherical resection 34 and to replace the articularsurface of the inferior facet 30 immediately inferior to it. Lithefixation member 52 may take the form of a pedicle screw designed to beimplanted in the corresponding pedicle 20 to anchor the implant 50 inplace. The orthopedic fastener 54 is designed to be coupled to thefixation member 52 to hold the implant 50 against the vertebral 12.

In the embodiment of FIG. 1, the implant 50 has a fixation portion 60,an articulation portion 62, and a stem 64. The fixation portion 60 isshaped to be attached to the semispherical resection 34, and thearticulation portion 62 provides a surface that articulates with anadjacent vertebral facet to carry out the function of the inferior facet30. The articulation portion 62 is coupled to the fixation portion 60 bythe stem 64.

As shown, the fixation portion 60 has a bone apposition surface 66,which may be generally semispherical to correspond to the shape of thesemispherical resection 34. The fixation portion 60 also has an aperture(not visible in FIG. 1) that passes through the bone apposition surface66 to receive the fixation member 52. The aperture is somewhat largerthan the exterior surface of the fixation member 52 so that the boneapposition surface 66 is able to slide against the semisphericalresection 34 with the fixation member 52 in place, implanted in thepedicle 20.

The articulation portion 62 similarly has an articulation surface 68designed to articulate with a superior facet of a vertebra immediatelyinferior to the vertebra 12. The articulation surface 68 may have aconvex shape, which may further be semispherical, semicylindrical, orthe like. The articulation surface 68 may be designed to articulate witha natural superior facet or a prosthetic superior facet.

In addition to the bone apposition surface, the fixation portion 60 alsohas an interpositional interface 70 shaped to interact with the fastener54 in a manner that will be described subsequently. The interpositionalinterface 70 has a generally conical surface that converges to theaperture of the fixation portion 60.

In the embodiment of FIG. 1, the fixation member 52 has a distal end 74implanted into one of the pedicles 20 of the vertebra 12, and a proximalend 76 that protrudes from the corresponding saddle point 32. The distalend 74 has threads (not visible in FIG. 1) that facilitate implantationof the distal end 74 in the pedicle 20 and keep the implanted distal end74 in place. The proximal end 76 has a plurality of threads 78 that areexposed to receive the fastener 54. Additionally, the proximal end 76has a torquing interface that may be used to apply torque to thefixation member 52 to implant the distal end 74 in the pedicle 20. Thetorquing interface may take the form of a hexagonal recess into which ahexagonal driver end can be inserted.

As shown in FIG. 1, the fastener 54 includes an interpositional member82 and a compression member 84. The interpositional member 82 may takethe form of a split ring, as illustrated. The compression member 84 isdesigned to be advanced along the proximal end 76 of the fixation member52 to press the interpositional member 82 into the interpositionalinterface 70 of the implant 50.

More specifically, the interpositional member 82 has an implantinterface 88, a compression interface 90, and a gap 92. As embodied inFIG. 1, the implant interface 88 includes a generally conical exteriorsurface designed to mate with the interpositional interface 70 in such amanner that the interpositional member 82 is compressed inwardrelatively uniformly as it is urged into the interpositional interface70. The compression interface 90 includes a generally semisphericalinterior surface designed to receive a corresponding surface of thecompression member 84. The gap 92 enables the interpositional member 82to obtain a relatively high degree of deflection to provide two verydistinct configurations: an unlocked configuration, in which theinterpositional member 82 is relatively undeflected, and a lockedconfiguration in which the interpositional member 82 is compressed tonarrow or remove the gap 92.

The interpositional member 82 may be formed of a material with arelatively low stiffness, such as plastic or rubber. A low stiffnessprovides relatively high deflection in the interpositional member 82without requiring excessive force. Accordingly, moving the fastener 54between the unlocked and locked configurations is relatively easilyaccomplished.

The compression member 84 has an interpositional interface 96, atorquing interface 98, and a bore 100. The interpositional interface 96is shaped to mate with the compression interface 90 of theinterpositional member 82. More specifically, the interpositionalinterface 96 may include an exterior, semispherical surface positionablewithin the compression interface 90. The torquing interface 98 maycomprise a castle nut interface, with a plurality of radial projectionsthat can be engaged by the end of a tool (not shown) with projectionsthat mesh with the torquing interface 98. The torquing interface 98 isable to receive torque from such a tool to enable the compression member84 to be advanced along the proximal end 76 of the fixation member 52.

The bore 100 passes through the compression member 84 and is sized toreceive the proximal end 76. The bore 100 has threads (not shown) thatengage the threads 78 of the proximal end 76 such that the compressionmember 84 advances along the proximal end 76 in response to rotation ofthe compression member 84.

The compression interface 90 and the interpositional interface 96 aresized such that they mate together with clearance when theinterpositional member 82 is relatively undeflected. Thus, theinterpositional member 82 and the compression member 84 are able torotate with respect to each other about all three axes 40, 42, 44.Accordingly, when the interpositional member 82 is relativelyundeflected or less deflected, the fastener 54 is in an unlockedconfiguration. When the interpositional member 82 is compressed, thecompression interface 90 tightly engages the interpositional interfaceso that relative rotation about all three axes 40, 42, 44 is restricted.Thus, when the interpositional member 82 is relatively compressed, thefastener 54 is in a locked configuration.

The interpositional interface 96 may have features, such as a pluralityof ridges 102, that are designed to engage the interpositional member 82in the locked configuration to enhance locking. The ridges 102 extendaround the circumference of the interpositional interface 96, about thefirst axis 40. Each of the ridges 102 has a faceted shape, as shown inFIG. 1, so that each ridge 102 has multiple relatively sharp projectionsthat extend outward form the generally semispherical shape of theinterpositional interface 96.

The compression member 84 may be formed of a material harder than thatof the interpositional member 82. For example, the compression member 84may be formed of a biocompatible metal. Accordingly, the projections ofthe ridges 102 may embed themselves into the compression interface 90when the fastener 54 is moved to the locked configuration to enhancelocking by resisting relative rotation between the interpositionalmember 82 and the compression member 84.

The interpositional interface 96 and the compression interface 90 may berelatively sized such that, in the unlocked configuration, there islittle enough clearance that the interpositional member 82 and thecompression member 84 will generally remain assembled. Thus, theinterpositional member 82 and the compression member 84 may bepre-assembled (i.e., factory assembled or the like), so that thefastener 54 is ready for use at the commencement of the surgicalprocedure without further assembly.

The fastener 54 may be used in concert with the fixation member 52 toretain the implant 50 in any of a plurality of orientations with respectto the vertebra 12. The manner in which this is carried out will beshown and described in greater detail with reference to FIGS. 2 and 3,as follows.

Referring to FIG. 2, a cephalad, section view illustrates the apparatus10 and the vertebra 12, with the apparatus 10 fully assembled and withthe fastener 54 in the unlocked c: is configuration. As shown, thedistal end 74 of the fixation member 52 has threads 104 that areembedded in the body 18 and in the corresponding pedicle 20 of thevertebra 12. The fixation portion 60 of the implant 50 has an aperture106 to which the generally conical surface of the interpositionalinterface 70 of the implant 50 converges. The bore 100 of thecompression member 84 has threads 108 that engage the threads 78 of theproximal end 76 of the fixation member 52. These elements were brieflydescribed in the discussion of FIG. 1, but were not visible in FIG. 1.

In order to install the implant 50, the posterior elements of thevertebra 12 may first be exposed through the use of procedures known inthe art. Measurements may be made to select the implant 50 and determinethe appropriate orientation for the implant 50, and any other implantsto be installed in the same operation. The semispherical resection 34may be formed via a reaming operation or the like, and other portions ofthe vertebra 12, such as the interior facet 30 to be replaced, may beresected as needed. The fixation member 52 may be implanted in thecorresponding pedicle 20 along the desired angle.

The implant 50 may then be inserted and positioned such that the boneapposition surface 66 abuts the semispherical resection 34. Prior toadjustment of the orientation of the implant 50, the fastener 54 may beinstalled so that it can be used to easily fix the implant 50 in placeonce it has reached the desired orientation. As mentioned previously,the interpositional member 82 and the compression member 84 may bepre-assembled, and therefore, they may not need to be assembled prior toimplantation.

The compression member 84, with the interpositional member 82 alreadycoupled to it, may be positioned so that the proximal end 76 of thefixation member 52 passes into the bore 100 of the compression member84. The compression member 84 may then be rotated to cause the threads108 of the bore 100 to engage the threads 78 of the proximal end 76. Theinterpositional member 82 is inserted into the interpositional interface70 of the implant 50 such that the generally conical implant interface88 of the interpositional member 82 engages the generally conicalsurface of the interpositional interface 70. The interpositional member82′ is thereby drawn into coaxiality with the fixation portion 60 of theimplant 50.

Further rotation of the compression member 84 will urge theinterpositional member 82 further along the interpositional interface70, toward the pedicle 20. The engagement of the generally conicalsurfaces of the interpositional interface 70 and the implant interface88 causes the interpositional member 82 to contract in response to suchmotion, thereby bringing the fastener 54 to the locked configuration.Until the implant 50 has been adjusted to the desired orientation, thecompression member 84 may be rotated just far enough to keep it in placeto facilitate subsequent tightening, but not far enough to move thefastener 54 to the locked configuration.

Thus, the apparatus 10 reaches the configuration shown in FIG. 2. Thefastener 54 is coupled to the proximal end 76 of the fixation member 52,but has not been tightened. Accordingly, although the orientation of theinterpositional member is substantially fixed with respect to theimplant 50, the compression member 84 is rotatable about any of the axes40, 42, 44 with respect to the interpositional member 82. Accordingly,the implant 50 is still relatively freely rotatable about the axes 40,42, 44 with respect to the vertebra 12.

The implant 50 may then be adjusted by rotating the implant 50 withrespect to the vertebra 12 such that the bone apposition surface 66rotates about any or all of the axes 40, 42, 44 within the semisphericalresection 34, until the articulation surface 68 is at the desiredposition and angle. Then, it is desirable to fix the orientation of theimplant 50 with respect to the bone, as will be shown and described inconnection with FIG. 3.

Referring to FIG. 3, a cephalad, section view illustrates the apparatus10 and the vertebra 12, with the fastener 54 in the lockedconfiguration. After rotational adjustment of the implant 50 withrespect to the vertebra 12, the compression member 84 is furtherrotated, for example, by engaging the torquing interface 98 with asuitable tool, as described previously, and applying torque. Thecompression member 84 is advanced along the proximal end 76, toward thepedicle 20 so that the interpositional member 82 is compressed byengagement with the interpositional interface 70 of the implant 50.

As the interpositional member 82 is compressed, the gap 92 shrinks andthe compression interface 90 of the interpositional member 82 contractsto grip the interpositional interface 96. The ridges 102, or just theprojections thereof, embed themselves in the compression interface 90 torestrict and/or entirely prevent further rotation of the compressionmember 84 with respect to the interpositional member 82. Theinterpositional interface 70 of the implant 50 and the implant interface88 of the interpositional member 82 engage each other in such a mannerthat relative rotation between the implant 50 and the interpositionalmember 82 is substantially prevented about the second and third axes 42,44. Relative rotation about the first axis 40 is also restricted as theinterpositional member 82 is compressed, and therefore, providesfrictional force as it presses outward against the interpositionalinterface 70.

Accordingly, when the compression member 84 is advanced along theproximal end 76 to provide the locked configuration, relative rotationbetween the vertebra 12 and the implant 50 are substantially prevented.This is the configuration illustrated in FIG. 3. The implant 50 islocked in its preferred orientation with respect to the vertebra 12. Theimplant 50 can be fixed in any of a wide variety of differentorientations with respect to the vertebra 12 through the use of thefixation member 52 and the fastener 54.

The embodiment shown in FIGS. 1, 2, and 3 is only one of manyembodiments of the present invention. In alternative embodiments,differently shaped implants, interpositional members, compressionmembers, and fixation members may be used. For example, generallyconical surfaces need not be used to interface between an implant and aninterpositional member. Such an interfacing surface may includecylindrical, semispherical, conical, parabolic, or other shapes, orcombinations thereof. According to one alternative embodiment, aninterfacing surface man include a cylindrical component adjoining asemicylindrical or parabolic component that flares the interface toprovide a diameter larger than that of the cylindrical component. Suchan interfacing surface may also have a polygonal cross section, or maybe keyed or otherwise shaped to limit an implant to a discrete number ofrelative orientations with respect to a vertebra.

Similarly, generally semispherical surfaces need not be used tointerface between the interpositional member and the compression member.Such interfacing surfaces may include cylindrical, semispherical,conical, parabolic, or other shapes, or combinations thereof. Bycontrast with the embodiment of FIGS. 1, 2, and 3, alternativeembodiments of the invention need not necessarily provide rotation of animplant with respect to a vertebra about three perpendicular axes.

In some alternative embodiments, additional features may be added toenhance locking provided by the locked configuration. Such features mayenable an implant to receive greater loads without moving from itspreferred orientation with respect to the vertebra. One example of suchan alternative embodiment will be shown and described in connection withFIG. 4, as follows.

Referring to FIG. 4, a perspective view illustrates an apparatus 110according to one alternative embodiment of the invention, used inconjunction with a vertebra 12, as described previously. As shown, theapparatus 110 includes an implant 150, a fixation member 52, and anorthopedic fastener 154, or fastener 154. The fixation member 52 may beidentical to that of the previous embodiment. The implant 150 and thefastener 154 are similar to their counterparts of previous embodiment,but include additional features to enhance locking, as will be describedbelow.

As shown, the implant 150 has a fixation portion 160 designed to beattached to the semispherical resection 34. The fixation portion 160 hasan interpositional interface 170, which has a generally conical surfacelike that of the interpositional interface 70 of the previousembodiment. However, the interpositional interface 170 does not providea smooth conical surface. Rather, the interpositional interface 170 hasa plurality of ridges 172 distributed along a generally radiallysymmetrical pattern about the generally conical surface.

The ridges 172 protrude inward to resist relative rotation between theimplant 150 and an interpositional member 182 of the fastener 154. Likethe interpositional member 82 of the previous embodiment, theinterpositional member 182 has an implant interface 88 with a generallyconical shape. The implant interface mates with the interpositionalinterface 170 of the implant 150. By contrast with the previousembodiment, the ridges 172 of the interpositional interface 170 are ableto penetrate the generally conical surface of the implant interface 88to restrict, or even substantially prevent, relative rotation betweenthe implant 150 and the interpositional member 182 about the first axis40.

Penetration of the implant interface 88 by the ridges 172 may occurbecause the implant 150 may be formed of a material that is harder thanthat of the interpositional member 182. For example, the interpositionalmember 182 may be formed of a polymer, elastomer, or the like, while theimplant 150 may be formed of a metal, a ceramic, or a harder polymer orelastomer. Thus, the ridges 172 may serve to enhance the lockingprovided by the locked configuration of the fastener 154.

In addition to the interpositional member 182, the fastener 154 has acompression member 84, which is identical to that of the previousembodiment. However, the interpositional member 182 differs from theinterpositional member 82 of the previous embodiment in that theinterpositional member 182 has a compression interface 190 with featuresdesigned to enhance locking with the interpositional interface 96 of thecompression member 84. More precisely, the compression interface 190includes a generally semispherical surface like that of the previousembodiment. However, the compression interface 190 has a plurality ofridges 192 that extend along generally circular pathways about thecircumference of the of the compression interface 190.

In the locked configuration, the ridges 192 enhance locking by helpingprevent rotation between the compression interface 190 and theinterpositional interface 96 of the compression member 84. Moreprecisely, the ridges 192 may deform against the interpositionalinterface 96 of the compression member 84, thereby providing regions inwhich the frictional force between the compression interface 190 and theinterpositional interface 96 is relatively large. The ridges 192 mayalso directly block motion of the ridges 102 of the interpositionalinterface 96, thereby particularly restricting relative rotation betweenthe interpositional member 182 and the compression member 84 about thesecond and third axes 42, 44.

Thus, the ridges 172 and the ridges 192 that have been added in theapparatus 110 of FIG. 4 may serve to enhance locking of the orientationof the implant 150 with respect to the vertebra 12, in the lockedconfiguration of the fastener 154. Those of skill in the art willrecognize that alternative features may be added to enhance locking. Inselected alternatives, the ridges 172 of the interpositional interface170 of the implant 150 may follow non-converging or curvilinear pathwaysdesigned to avoid interfering with compression of the interpositionalmember 182. Furthermore, the differently-shaped surfaces describedpreviously for interfacing between the implant 50 and theinterpositional member 82 or between the interpositional member 82 andthe compression member 84 may be used in conjunction with features likethe ridges 172 and the ridges 192 of the embodiment of FIG. 4 to provideenhanced locking for such alternative interfacing surface shapes.

The present invention has particular relevance to orthopedic medicine,and more particularly to facet joint replacement. However, theprinciples, structures, and methods of the present invention may also beextended to a wide variety of other fields. The present invention may beembodied in other specific forms without departing from its spirit oressential characteristics. As such the described embodiments are to beconsidered in all respects only as illustrative and not restrictive. Thescope of the invention is, therefore, indicated by the appended claimsrather than by the foregoing description. All changes which come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

1. An apparatus comprising: a prosthesis comprising a bone contactingsurface comprising a three-dimensional section of a sphere and anarticular surface having a generally planar surface shaped to replace anatural articular surface of a bone; and an orthopedic fastenerconfigured to secure the prosthesis to a bone, the orthopedic fastenercomprising: a compression member comprising a bore defining a firstcenter axis; and an interpositional member comprising a split ringcomprising a flat circular end comprising a center point, and a secondcenter axis extending perpendicular to the flat circular end and throughthe center point, wherein the interpositional member cooperates with thecompression member to provide an unlocked configuration in which theinterpositional member is rotatable with respect to the compressionmember such that the first axis is capable of being selectively orientedto parallel and nonparallel positions relative to the second axis, and alocked configuration in which the second axis is maintained in itsselected parallel or nonparallel orientation relative to the first axisand relative rotation between the interpositional member and thecompression member is restricted.
 2. The apparatus of claim 1, furthercomprising a fixation member implantable in the bone, the fixationmember having a receiving interface configured to cooperate with thecompression member to urge the compression member toward the bone. 3.The apparatus of claim 2, wherein the fixation member comprises a distalend shaped to be implanted in the bone, and a proximal end havingthreads, wherein the compression member comprises threads shaped tocooperate with the threads of the proximal end to cause the compressionmember to advance toward the distal end in response to rotation of thecompression member with respect to the fixation member.
 4. The apparatusof claim 1, wherein the bone comprises a vertebra.
 5. The apparatus ofclaim 4, wherein the articular surface is shaped to replace a naturalarticular surface of a facet of the vertebra.
 6. The apparatus of claim1, wherein the bone contacting surface comprises a generallysemispherical shape.
 7. The apparatus of claim 1, wherein at least oneof the compression member and the interpositional member is configuredto deform in response to motion of the compression member toward thebone to lock against the other of the compression member and theinterpositional member.
 8. The apparatus of claim 7, wherein at leastone of the compression member and the interpositional member comprises aplurality of features that protrude toward the other of the compressionmember and the interpositional member to restrict relative rotationbetween the compression member and the interpositional member.
 9. Theapparatus of claim 7, wherein the interpositional member comprises asplit ring configured to contract in response to motion of thecompression member toward the bone.
 10. The apparatus of claim 1,wherein the interpositional member comprises a prosthesis interfacehaving a generally conical shape, wherein the prosthesis comprises aninterpositional interface having a generally conical shape that mateswith the prosthesis interface to restrict rotation of the prosthesiswith respect to the bone about the second axis.
 11. The apparatus ofclaim 10, wherein at least one of the prosthesis interface and theinterpositional interface comprises a plurality of features thatprotrude toward the other of the prosthesis interface and theinterpositional interface to restrict relative rotation between theprosthesis and the interpositional member.
 12. The apparatus of claim 1,wherein the orthopedic fastener is configured to move from the unlockedconfiguration to the locked configuration in response to urging of thecompression member toward the bone to hold the three-dimensional sectionagainst the bone.
 13. An apparatus comprising: a prosthesis comprisingan articular surface shaped to replace a natural articular surface of afacet of a vertebra and configured and dimensioned to articulate with anatural articular surface of an adjacent facet; an orthopedic fastenerconfigured to secure the prosthesis to the vertebra, the orthopedicfastener comprising: a compression member; and an interpositional membercomprising a locking surface, a prosthesis interface, a flat circularend extending between the locking surface and the prosthesis interfaceand comprising a center point, and a center axis, wherein the centeraxis is perpendicular to the flat circular end and passes through thecenter point, wherein the locking surface engages the compression memberto provide an unlocked configuration in which the interpositional memberis rotatable with respect to the compression member, and a lockedconfiguration in which relative rotation between the interpositionalmember and the compression member is restricted, and the prosthesisinterface is shaped to engage the prosthesis to press the prosthesistoward the vertebra in response to urging of the compression membertoward the vertebra; and a fixation member implantable in the vertebra,wherein the fixation member is adapted and configured to be selectivelyoriented to parallel and nonparallel orientations relative to the centeraxis in the unlocked configuration, and the fixation member isselectively fixed in a parallel or nonparallel orientation relative tothe center axis in the locked configuration, the fixation member havinga receiving interface configured to cooperate with the compressionmember to urge the compression member toward the vertebra.
 14. Theapparatus of claim 13, wherein the orthopedic fastener is configured tomove from the unlocked configuration to the locked configuration inresponse to urging of the compression member toward the vertebra. 15.The apparatus of claim 13, wherein the fixation member comprises adistal end shaped to be implanted in the vertebra, and a proximal endhaving threads, wherein the compression member comprises threads shapedto cooperate with the threads of the proximal end to cause thecompression member to advance toward the distal end in response torotation of the compression member with respect to the fixation member.16. The apparatus of claim 13, wherein at least one of the compressionmember and the interpositional member is configured to deform inresponse to motion of the compression member toward the vertebra to lockagainst the other of the compression member and the interpositionalmember.
 17. The apparatus of claim 16, wherein at least one of thecompression member and the interpositional member comprises a pluralityof features that protrude toward the other of the compression member andthe interpositional member to restrict relative rotation between thecompression member and the interpositional member.
 18. The apparatus ofclaim 13, wherein the interpositional member comprises an prosthesisinterface having a generally conical shape, wherein the prosthesiscomprises an interpositional interface having a generally conical shapethat mates with the prosthesis interface to restrict rotation of theprosthesis with respect to the vertebra about the axis.